With progress of speech processing and natural language processing in recent years, a speech translation apparatus is being developed. In communicating different languages, the speech translation apparatus translates a source language sentence spoken by a source language user to a target language sentence and synthesizes a target language speech to a target language user.
The conventional speech translation apparatus outputs the synthesized speech, regardless of inputting another speech or not. And the output of the synthesized speech and the other speech are overlapped. Therefore mistakes of communication between users occur. The mistakes are frequently encountered in cases of non-face-to-face communication, for example, by telephone with the conventional speech translation apparatus.